A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The throughput of a lithographic apparatus, measured in substrates (wafers) processed per unit of time, depends, inter alia, from the velocity, acceleration and jerk with which movable parts of the lithographic apparatus may be displaced by servo motors, and the settling time of these movable parts, where the settling time is the total elapsed time just after completing a position or velocity setpoint until the servo error is within a required position and/or velocity accuracy window. Examples of devices comprised in a lithographic apparatus and having such movable parts are substrate handling devices and reticle handling devices. A displacement of the movable, parts generally is considered to take place substantially in x and y directions while irradiating target portions of a substrate.
An increase of the throughput may be obtained by increasing the force generated by an actuator assembly driving a movable part so as to increase the acceleration, and by reducing the weight of the movable part as much as possible. A suitable structure of a movable part having a low weight is a plate-like structure supported by ribs. During high acceleration such a structure, however, may be susceptible to internal structural elastic deformation, resulting in an unacceptable cross-talk between different degrees of freedom that are controlled. Thus, with a deformation during high acceleration, a large position error may occur that takes a long settling time, and thus resulting in a loss of potential throughput (e.g. measured in wafers/hour).
In general terms, a lithographic stage or servo positioning performance is expressed as a time Moving Average error (MA error) and a time Moving Standard Deviation (MSD) of the error. A critical time window here is the time interval that each point on a die is exposed (in other words: receives photons). If the average position error for a point on the die during this time interval is high (in other words: high MA-error), the effect is a shift of the exposed image, resulting in overlay errors. If the standard deviation of the position error during this time interval is high (in other words: high MSD error), the image may smear, resulting in fading errors.